Chromium-plating electrolyte



Patented Nov. 8, 1938 UNITED STATES 2,136,197 OHBOMIUM-PLATING ELECTROLTIE Robert W. Shaffer, Kenosha, Win, asaignor tov Snap-On Tools, Inc., Kenosha, Wia, a corporation of Delaware No Drawing. Application September 25, 1936,

Serial No. 102,599

'lClalms.

This invention relates'to the electro -deposition Still another object is to provide a new and of chromium and more particularly to the preparation of an improved electrolyte solution for use in connection therewith.

It contemplates more especially. the provision of a more stable electrolyte chromium solution to effect chromium plating without resort to sensitive and constant regulation which, is incident to the use of known catalytic agents preferably comprising acids and bases having sulphate, fluoride, phosphate and borate radicals.

Science has for many years sought a commercially practical chromium plating process and solution, and the art abounds with the suggested use of chromic acid as an electrolytehaving a stable catalytic agent included therewith. Catalytic agents comprising acids and bases having sulphate, fluoride, phosphate and borate radicals have been suggested by Fink in United States Letters Patent numbered 1,581,188. standing this disclosure as an available process of electro-depositing chromium, yet such catalysts and electrolytes present a requirement for constant and skilled supervision attended with sensitive and frequent proportional restoration of the catalyst and chromic acid in solution within definite and comparatively narrow limits and practical ranges to meet commercial requirements.

Unless the proportional concentration of the catalysts to the chromic acid in solution is maintained within definite ascertained limits, the results with known processes and electrolytes are always uncertain and unreliable within the requirements of a satisfactory commercial process. This entails constant chemical tests and skilled supervision that is expensive and retards production. Sensitive electrolytes of known composition forchromium plating are not, therefore, capable of general use unless skilled supervision is available andfrequent tests are made for maintaining the requisite regulation and solution concentration. Then, too, chemical tests of the required character for maintaining known electrolytes within practical ranges of concentration for successful chromium plating, are tedious and not ordinarily within the capacity of practical plating experts who have, forthe most part, very little knowledge. of chemistry andchemical reactions;

' One object of the present invention is to provide a new and improved electrolyte for the cathodic deposition of chromium in the metallic state.

Another object is to provide an improved chroinium electrolyte which is not appreciably sensitive in the proportional concentration of stable or unstable radicals to the chromic acid in the solution" for the commercially successful cathodic deposition of chromium.

Notwithimproved electrolyte embodying catalysts having a comparatively wide range of concentration with the chromic acid in solution for the cathodic deposition of chromium in a metallic state.

A further object is to provide a catalyst for a chromium electrolyte that is comparatively insoluble therein, increases the throwing power, and is capable of extended use therewith without requiring frequent replenishment for efiecting the cathodic deposition of chromium in a metallic state.

A still further object is to provide an improved stable catalyst such as manganese dioxide that has a wide range of concentration with chromic acid in combination with unstable radicals to aiford the commercially successful cathodic deposition of chromium in a metallic state.

Still a further object is to provide a chromium electrolyte with a stable catalyst such as manganese dioxide in combination with unstable radicals such as manganese chloride to maintain an operative solution for the cathodic deposition of chromium with increased throwing power.

Other objects and advantages will appear from the following description of an illustrative embodiment of the present invention.

In practicing my invention, electric current is passed through the electrolyte from an anode that mayconsist of lead to a cathode or cathodes which are objects-of electrically conductive material whose surfaces are to receive the deposit of chromium in a metallic state from an electrolytic solution preferably comprising chromic acid and a stable insoluble catalyst in combination with unstable radicals having a comparativel'y wide range of concentration therewith;

The catalysts which I have found most effective andldesirable and which is appreciably insoluble in the chromium carrying electrolyte is manganese dioxide. Manganese'dioxide has a wide range of concentration therewith ranging from approximately one gram of manganese dioxide per liter to super-saturation whlch in actual test has even exceeded 500 grams per liter to 150 grams on chromic acid (anhydride) per liter. -'I.'he standard concentration of chromic acid is approximately grams per liter. dioxide is utilized in combination with potassium Manganese permanganate and manganese chloride which are comparatively unstable radicals. To avoid the formation of a brown scum and to improve the character of the plating, I have found that it is highly desirable to use a comparatively small quantity of Rochelle salts or sodium potassium tartrate:

Manganese dioxide is neither a salt nor an acid nor does it combine with the chromic acid, but merely aids in the chemical action without being itself permanently altered. Except for mechanical losses and the slight exhaustion of the oxygen content therefrom which is attendant with its settling to the bottom of the bath, this manganese catalyst can be removed from the bath, washed clean, and then exposed to the air for a short time, approximately an hour, preparatory to reuse as originally prescribed in the form of manganese dioxide. This catalytic agent which is prescribed herein and preferred in solution with chromic acid, is operative within a wide range of concentration therewith.

Manganese dioxide is stable in the bath and remains substantially stable under the electrolytic action, and is believed responsible for improved and more eifective throwing power of metallic chromium upon the work when the current is passed through the bath for the cathodic deposition of chromium in a metallic state. Excess manganese dioxide is preferably present in the bath at all times. Such unstable radicals as chloride and potassium permanganate are also included with the manganese dioxide and chromic acid. This composition is not highly sensitive and does not require such frequent replenishment or regulation such as acids and bases having a sulphate radical, acids and bases having a fluoride radical, acids and bases which have a borate radical of the character prescribed and claimed in the Fink patent numbered 1,581,188.

In fact, manganese dioxide need not be limited in its concentration with chromic acid, since a wide range of concentration varying from as low as one gram per liter to an almost unlimited concentration is operable in that an excess concentration of 500 grams or more of manganese dioxide has given satisfactory results. The quantity of Rochelle salts which should be used can not be well stated in terms of weights and volumes because it is a function of such factors ascurrent density, temperature of bath, concentration of chromic acid, proportion of manganese chloride or other compound added to the chromic acid, the anode material and the shape of the object to be plated. Therefore the amount of Rochelle salts which is believed to be the filmpreventing substancethat should be added to the electrolyte can best be found by trial, although it may be stated briefly that the amount used should be that quantity which is just necessary to prevent the formation of a brown film on the cathode when the operator is working with a clean piece of work 'such as an object of brass, steel or nickel.

If substantially more than this amount is added, the efliciency of the plating process is appreciably diminished. As an example of a specific electrolyte suitable for commercial requirements and the process of preparing the solution, the following formula is given:

Manganese dioxide (Russian imported preferre ounds" 40 Potassium permanganate do 15 Mangane e chloride (in 5 Rochelle salts (sodium potassium tartrate) ds 5 Chromic acid..- do 500 Water gallons..- 400 The above proportions are not criticaland may be initially utilized to properly charge a 450 gallon plating tank. In use, the chromic acid content is maintained at approximately grams per liter. Sensitive regulation by analysis of the solution has been found unnecessary in that commercially successful cathodic deposition of chromium in a metallic state can be efiected until the cathodic deposition of chromium becomes unsatisfactory whereupon the aforesaid ingredients with the exception of manganese dioxide are replaced. Should a brown scum appear on the cathodes or objects to be plated, such evidences that there is a need for additional Rochelle salt and that additional chromic acid must be added or supplied should the cathodic deposition of chromium still remain unsatisfactory. These indicators are capable of visual ascertainment and recognition so that chemical analyses are largely dispensed with and the hath made operative by adding to but never subtracting from the total quantity of stable and/or unstable catalytic agent already present in the chromic acid solution from the previous composition and operation. Poor throwing power calls for the addition of manganese dioxide preferably of Russian origin. Commercial hydrochloric acid may be substituted for manganese chloride, but the latter is preferred.

Regulation within said given limits for continuous operation is not significant in that the greater the concentration, the longer the electrolyte is operable for the cathodic deposition of chromium in a metallic state for quantity productions; therefore, regulation and chemical analyses are largely dispensed with and more ingredients are added only when a brown scum forms upon the cathode or the objects of plating are satisfactorily covered with a clear film of metallic chromium.

As stated the general amount of catalytic agent in the bath when utilizing manganese dioxide should not be less than 1 gram per liter, but there is no recognizable maximum for effecting operable results in the cathodic deposition of chromium in a metallic state. 01 course, varying proportions of manganese dioxide per liter of chromic acid will prove most effective when different types of work or surfaces serve as the cathode. for large surfaces such as a flat plate of extended area will require a solution of increased throwing power, that is a stronger minimum portion of manganese dioxide per liter of chromic acid, but on small objects such as hand tools it has been found that approximately fifty (50) grams of manganese dioxide per liter of a solution containing one hundred and fifty grams vper liter of chromic acid is most satisfactory. In the main, the electrolyte is supersaturated with manganese dioxide and the excess gradually utilized over extended periods of cathodic deposition of metallic chromium.

While the finest obtainable commercial chromic acid (anhydride) is desirable for use in connection with manganese dioxide, yet ordinary commercial grades of chromic acid is operable and has been successfully used. It is said that usual current densities that are resorted to for such electrolytes are utilizable with the afore-"l cathodic deposition of chi'on'ilum in a metallic,

state without requiring any appreciable regulation or chemical analyses that have heretofore been indispensable in known processes of electrodepositing chromium and the preparation of electrolytes therefor.

Various changes may be made in the embodiment of the invention herein specifically described without departing from or sacrificingany of the advantages of the invention or any features thereof, and nothing herein shall be con-' strued as limitations upon the invention, its concept or structural embodiment as to the whole or any part thereof except as defined in the appended claims.

I claim:

1. A method of electrodepositing chromium from solution of chromic acid which consists in the reduction of chromic acid at the cathode in the presence of manganese dioxide, potassium permanganate, manganese chloride, and Rochelle salts by passing an electrical current through the solution containing said ignredi'ents to eiiect the cathodic deposition of chromium in a metallic state.

2. A method 01 electrodepositing chromium from solution of chromic acid which consists in reducing the chromic acid at the cathode in the presence of manganese chloride and Rochelle salts in solution by passing an electrical current through said solution.

3. A method of electrodepositing chromium from a solution of chromic acid which consists in reducing the chromic acid at the cathode in the presence of potassium permanganate and Rochelle salts in solution by passing an electrical current therethrough.

4. A method of electrodepositing chromium from a solution of chromic acid which consists in reducing the chromic acid at the cathode with manganese chloride, potassium permanganate, and Rochelle salts, by passing an electrical current through said solution to effect the cathodic deposition of chromium in a metallic state.

5. An electrolyte for use in the cathodic deposition of chromium in a metallic state which comprises chromic acid having a concentration of approximately 130 grams per liter admixed with manganese dioxide, manganese chloride, potassium permanganate, and Rochelle salts.

6. A chromium electrolyte comprising chromic acid having a concentration of approximately 130 grams per liter, a substantial quantity of manganese dioxide, potassium' permanganate, manganese chloride and a comparatively small quantity of Rochelle salts.

'7. A chromium electrolyte comprising chromic acid having an approximate concentration of 130 grams per liter, substantially- 40 pounds of manganese dioxide, 15 pounds of potassium permanganate, 5 pounds manganese chloride, 5 pounds Rochelle salts and 400 gallons of water.

ROBERT w. SHAFFER. 

